PETROGRAPHIC CLUES TO THE RICHMONDIAN INVASION OF LIMESTONES FROM THE UPPER ORDOVICIAN (UPPER KATIAN) FROM THE MADISON, INDIANA AREA

Limestone-mudstone bundles or cycles within the Cincinnatian C5 sequence in southeastern Indiana record the “Richmondian Invasion”. Our study attempts to provide a finer-resolution look at the environmental conditions of the upper part of the strata that record this biotic turnover. Two sections (roadcuts on US Highway 421—one north of Madison, IN; one in Milton, KY), separated by over six kilometers, were measured and described at cm resolution and a gamma ray spectrometer was used to log each section at 0.25 m spacing. This allowed for a precise correlation between the two sections, enabling the generation of a composite section. Over 200 limestone samples were collected from these outcrops. We estimated fossil abundances from the surfaces of each slab. The slabs were then cut, polished, and used to make nearly 300 thin sections. Some remaining material was set aside for acetic acid digestion (for microfossils) and for XRF analysis. Preliminary cluster analysis and ordination of faunal abundance data, when plotted against the composite section, displayed a shift in fossil assemblages. Thin sections show a variety of fossils, arrangements, rock types, matrices, and cements. Longer seafloor exposure and repeated reworking are suggested by various degrees of skeletal fragmentation. Other notable features in the thin sections include phosphate content, pyritization, erosional surfaces, and microborings. Quantifying these features provides further clues regarding upper Richmondian environmental conditions (e.g., correlations between rock type and phosphate content or between microborings and water depth). Thin sections also allow for bryozoan identification. XRF analysis data, when plotted stratigraphically, elucidates the relationships between various elements. The correlation of phosphorus with rare earth elements offers insights into ocean chemistry and its influence on phosphate content. Our composite section suggests a large-scale marine regression which includes intermittent appearances and disappearances of various taxa, likely reflecting extinctions or migrations. Analysis is underway to combine this information with taphonomic and facies data from thin sections, which may shed light on the reasons for these faunal changes (e.g., the influence of sea level).